Thought I'd show you the "dash" lighting I'm considering. Left to right: hi beam, turn and neutral. Got them from the hot rod company.
Jeff

Started on the rear arm. Here are the YouTube videos.
https://www.youtube.com/watch?v=7giFOIKXN5Y
https://www.youtube.com/watch?v=pVb3vFlWkk4
https://www.youtube.com/watch?v=GOwcvuBNTB8
https://www.youtube.com/watch?v=yF8-wHSHaaA
Jeff

Jeff:

I looked at your video a bit.....not all of it....just scanned through for the highlights.

You are cutting away an awful lot of the brace, and taking a lot of strength away form the swing arm.  That cross brace is a structural piece, and does serve a purpose......and as with any structural piece taking away 1/2 the dimension reduces the strength by a factor of 4.

You might want to consider adding some thickness to the front of the brace to restore the strength you cut away.  There is room in the front to add material without making clearance problems.

I cut away far less than you did, and I added an internal stiffener to help keep the piece near the original strength.

I slid the axle all the way forward and marked a pattern using a 3/8" drive socket extension for clearance.


I then cut out the original metal, and made an interior stiffener to be welded inside, and a new plate to cover the part I cut away.  The stiffener plate I used is sitting on the top of the swing arm and is installed about where the green tape is located.


I then welded it all together and made it smooth with my grinder.  You can see the places where the welds show through where I attached the inner stiffening plate.


Also....before you paint and make it pretty, check the distance between the rear mounts of the swing arm.  I found that the welding process pulled the swing arm closer together at the rear, and I had to put a hydraulic jack between them to bend them back to the proper spacing to get the wheel back in place.
 

There are several things involved.

You are removing material....Not a huge deal.
You are reducing the distance between the outside members, which compresses the available material into a smaller package.  This reduces the Moment of Inertia. Take a 2x4 board and lay it flat....and it bends a lot.  Take that same 2x4 and stand it upright....and it becomes stiff.

The Moment of Inertia for a rectangular object is easy and can be calculated as 1/12 the base x the height cubed.  The 2x4 flat would have a moment of inertia of 2.67.  The same 2x4 stood upright would have a value of 10.67.....or 3.9950 times as strong.  Irregular shaped objects are a bit harder to calculate...but the same concept applies.  That is why a hollow tube is so much stronger than a steel bar of the same weight.

If you want to investigate this stuff....look up Moment of Inertia and Radius of Gyration.

Now the swing arm is not quite that simple, as it pivots from the front, has a welded in cross brace, has shocks mounted near the rear, and has a somewhat rigid attachment at the rear where the axle passes through - so the cross brace is not independent and functions as a very important stiffener.  If the cross brace is weak the swing arm can twist during cornering....the top of the tire will twist upward away from the paving.

my kindle has gotten real slow to type on.. I'd ask more.. thanks bunches..
Design, structural strength, I just Like that stuff. I stare at bridges trying to understand the WHY of all the braces..
The rule you quoted applies in several places. If I'm tracking right..
Heat transfer / distance
Cell phone exposure/ distance

Nonlinear,  

On the original 747 wing load test, they bent the wings back until the tips almost touched before failure.


Like to see a pic of THAT!
What a fun job...

http://www.supertightstuff.com/wp-content/uploads/2010/06/boeing_787_ultimate...

Can't find the 747, but WOW,  I wonder what kinda G forces would be needed to do that,,

Dave wrote on 03/16/15 at 09:47:51:

roflmao, which is pretty hard to stifle due to work conditions.

WWI, the load test was done inverted and they just piled dry sand on the wings.